Ca2+-CaM Dependent Inactivation of RyR2 Underlies Ca2+ Alternans in Intact Heart

Identifying Key Binding Interactions Between the Cardiac L-Type Calcium Channel and Calmodulin Using Molecular Dynamics Simulations

Defects in the binding of the calcium sensing protein calmodulin (CaM) to the L-type calcium channel (CaV1.2) or to the ryanodine receptor type 2 (RyR2) can lead to dangerous cardiac arrhythmias with distinct phenotypes, such as long-QT syndrome (LQTS) and catecholaminergic ventricular tachycardia (CPVT). Certain CaM mutations lead to LQTS while other mutations lead to CPVT, but the mechanisms by which a specific mutation can lead to each disease phenotype are not well-understood. In this study, we use long, 2 μs molecular dynamics simulations and a multitrajectory approach to identify the key binding interactions between the IQ domain of CaV1.2 and CaM. Five key interactions are found between CaV1.2 and CaM in the C-lobe, 1 in the central linker, and 2 in the N-lobe. In addition, while 5 key interactions appear between residues 120-149 in the C-lobe of CaM when it interacts with CaV1.2, only 1 key interaction is found within this region of CaM when it interacts with the RyR2. We show that this difference in the distribution of key interactions correlates with the known distribution of CaM mutations that lead to LQTS or CPVT. This correlation suggests that a disruption of key binding interactions is a plausible mechanism that can lead to these two different disease phenotypes.

The Dysfunction of Ca2+ Channels in Hereditary and Chronic Human Heart Diseases and Experimental Animal Models

Chronic heart diseases, such as coronary heart disease, heart failure, secondary arterial hypertension, and dilated and hypertrophic cardiomyopathies, are widespread and have a fairly high incidence of mortality and disability. Most of these diseases are characterized by cardiac arrhythmias, conduction, and contractility disorders. Additionally, interruption of the electrical activity of the heart, the appearance of extensive ectopic foci, and heart failure are all symptoms of a number of severe hereditary diseases. The molecular mechanisms leading to the development of heart diseases are associated with impaired permeability and excitability of cell membranes and are mainly caused by the dysfunction of cardiac Ca2+ channels. Over the past 50 years, more than 100 varieties of ion channels have been found in the cardiovascular cells. The relationship between the activity of these channels and cardiac pathology, as well as the general cellular biological function, has been intensively studied on several cell types and experimental animal models in vivo and in situ. In this review, I discuss the origin of genetic Ca2+ channelopathies of L- and T-type voltage-gated calcium channels in humans and the role of the non-genetic dysfunctions of Ca2+ channels of various types: L-, R-, and T-type voltage-gated calcium channels, RyR2, including Ca2+ permeable nonselective cation hyperpolarization-activated cyclic nucleotide-gated (HCN), and transient receptor potential (TRP) channels, in the development of cardiac pathology in humans, as well as various aspects of promising experimental studies of the dysfunctions of these channels performed on animal models or in vitro.

Bacterial toxins and heart function: heat-labile Escherichia coli enterotoxin B promotes changes in cardiac function with possible relevance for sudden cardiac death

Bacterial toxins can cause cardiomyopathy, though it is not its most common cause. Some bacterial toxins can form pores in the membrane of cardiomyocytes, while others can bind to membrane receptors. Enterotoxigenic E. coli can secrete enterotoxins, including heat-resistant (ST) or labile (LT) enterotoxins. LT is an AB5-type toxin that can bind to specific cell receptors and disrupt essential host functions, causing several common conditions, such as certain diarrhea. The pentameric B subunit of LT, without A subunit (LTB), binds specifically to certain plasma membrane ganglioside receptors, found in lipid rafts of cardiomyocytes. Isolated guinea pig hearts and cardiomyocytes were exposed to different concentrations of purified LTB. In isolated hearts, mechanical and electrical alternans and an increment of heart rate variability, with an IC50 of ~0.2 μg/ml LTB, were observed. In isolated cardiomyocytes, LTB promoted significant decreases in the amplitude and the duration of action potentials. Na+ currents were inhibited whereas L-type Ca2+ currents were augmented at their peak and their fast inactivation was promoted. Delayed rectifier K+ currents decreased. Measurements of basal Ca2+ or Ca2+ release events in cells exposed to LTB suggest that LTB impairs Ca2+ homeostasis. Impaired calcium homeostasis is linked to sudden cardiac death. The results are consistent with the recent view that the B subunit is not merely a carrier of the A subunit, having a role explaining sudden cardiac death in children (SIDS) infected with enterotoxigenic E. coli, explaining several epidemiological findings that establish a strong relationship between SIDS and ETEC E. coli.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12551-023-01100-6.

Targeting cAMP signaling compartments in iPSC-derived models of cardiovascular disease

Adenosine 3',5'-cyclic monophosphate (cAMP) acts as a second messenger that is involved in the regulation of a plethora of processes. The activation of cAMP signaling in defined compartments is critical for cells to respond to an extracellular stimulus in a specific manner. Rapid advances in the field of human induced pluripotent stem cells (iPSCs) reflect their great potential for cardiovascular disease modeling, drug screening, regenerative and precision medicine. This review discusses cAMP signaling in iPSC-derived cardiovascular disease models, and the prospects of using such systems to elucidate disease mechanisms, drug actions and to identify novel drug targets for the treatment of cardiovascular diseases with unmet medical need, such as hypertension and heart failure.

Calcium oscillation on homogeneous and heterogeneous networks of ryanodine receptor

Calcium oscillation is an important calcium homeostasis, imbalance of which is the key mechanism of initiation and progression of many major diseases. The formation and maintenance of calcium homeostasis are closely related to the spatial distribution of calcium channels on endoplasmic reticulum, whose complex structure was unveiled by recent observations with superresolution imaging techniques. In the current paper, a theoretical framework is established by abstracting the spatial distribution of the calcium channels as a nonlinear biological complex network with calcium channels as nodes and Ca^{2+} as edges. A dynamical model for a ryanodine receptor (RyR) is adopted to investigate the effect of spatial distribution on calcium oscillation. The mean-field model can be well reproduced from the complete graph and dense Erdös-Rényi network. The synchronization of RyRs is found important to generate a global calcium oscillation. Below a critical density of the Erdös-Rényi or BaraBási-Albert network, the amplitude and interspike interval decrease rapidly with the end of disappearance of oscillation due to the desynchronization. The clique graph with a cluster structure cannot produce a global oscillation due to the failure of synchronization between clusters. A more realistic geometric network is constructed in a two-dimensional plane based on the experimental information about the RyR arrangement of clusters and the frequency distribution of cluster sizes. Different from the clique graph, the global oscillation can be generated with reasonable parameters on the geometric network. The simulation also suggests that existence of small clusters and rogue RyRs plays an important role in the maintenance of global calcium oscillation through keeping synchronization between large clusters. Such results support the heterogeneous distribution of RyRs with different-size clusters, which is helpful to understand recent observations with superresolution nanoscale imaging techniques. The current theoretical framework can also be extent to investigate other phenomena in calcium signal transduction.

RyR2 Serine-2030 PKA Site Governs Ca2+ Release Termination and Ca2+ Alternans

BACKGROUND

PKA (protein kinase A)-mediated phosphorylation of cardiac RyR2 (ryanodine receptor 2) has been extensively studied for decades, but the physiological significance of PKA phosphorylation of RyR2 remains poorly understood. Recent determination of high-resolution 3-dimensional structure of RyR2 in complex with CaM (calmodulin) reveals that the major PKA phosphorylation site in RyR2, serine-2030 (S2030), is located within a structural pathway of CaM-dependent inactivation of RyR2. This novel structural insight points to a possible role of PKA phosphorylation of RyR2 in CaM-dependent inactivation of RyR2, which underlies the termination of Ca²⁺ release and induction of cardiac Ca²⁺ alternans.

METHODS

We performed single-cell endoplasmic reticulum Ca²⁺ imaging to assess the impact of S2030 mutations on Ca²⁺ release termination in human embryonic kidney 293 cells. Here we determined the role of the PKA site RyR2-S2030 in a physiological setting, we generated a novel mouse model harboring the S2030L mutation and carried out confocal Ca²⁺ imaging.

RESULTS

We found that mutations, S2030D, S2030G, S2030L, S2030V, and S2030W reduced the endoplasmic reticulum luminal Ca²⁺ level at which Ca²⁺ release terminates (the termination threshold), whereas S2030P and S2030R increased the termination threshold. S2030A and S2030T had no significant impact on release termination. Furthermore, CaM-wild-type increased, whereas Ca²⁺ binding deficient CaM mutant (CaM-M [a loss-of-function CaM mutation with all 4 EF-hand motifs mutated]), PKA, and Ca²⁺/CaMKII (CaM-dependent protein kinase II) reduced the termination threshold. The S2030L mutation abolished the actions of CaM-wild-type, CaM-M, and PKA, but not CaMKII, in Ca²⁺ release termination. Moreover, we showed that isoproterenol and CaM-M suppressed pacing-induced Ca²⁺ alternans and accelerated Ca²⁺ transient recovery in intact working hearts, whereas CaM-wild-type exerted an opposite effect. The impact of isoproterenol was partially and fully reversed by the PKA inhibitor N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinoline-sulfonamide and the CaMKII inhibitor N-[2-[N-(4-chlorocinnamyl)-N-methylaminomethyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulfonamide individually and together, respectively. S2030L abolished the impact of CaM-wild-type, CaM-M, and N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinoline-sulfonamide-sensitive component, but not the N-[2-[N-(4-chlorocinnamyl)-N-methylaminomethyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulfonamide-sensitive component, of isoproterenol.

Cardiac Alternans: From Bedside to Bench and Back

Cardiac alternans arises from dynamical instabilities in the electrical and calcium cycling systems of the heart, and often precedes ventricular arrhythmias and sudden cardiac death. In this review, we integrate clinical observations with theory and experiment to paint a holistic portrait of cardiac alternans: the underlying mechanisms, arrhythmic manifestations and electrocardiographic signatures. We first summarize the cellular and tissue mechanisms of alternans that have been demonstrated both theoretically and experimentally, including 3 voltage-driven and 2 calcium-driven alternans mechanisms. Based on experimental and simulation results, we describe their relevance to mechanisms of arrhythmogenesis under different disease conditions, and their link to electrocardiographic characteristics of alternans observed in patients. Our major conclusion is that alternans is not only a predictor, but also a causal mechanism of potentially lethal ventricular and atrial arrhythmias across the full spectrum of arrhythmia mechanisms that culminate in functional reentry, although less important for anatomic reentry and focal arrhythmias.

Calmodulin variant E140G associated with long QT syndrome impairs CaMKIIδ autophosphorylation and L-type calcium channel inactivation

Long QT syndrome (LQTS) is a human inherited heart condition that can cause life-threatening arrhythmia including sudden cardiac death. Mutations in the ubiquitous Ca²⁺-sensing protein calmodulin (CaM) are associated with LQTS, but the molecular mechanism by which these mutations lead to irregular heartbeats is not fully understood. Here, we use a multidisciplinary approach including protein biophysics, structural biology, confocal imaging, and patch-clamp electrophysiology to determine the effect of the disease-associated CaM mutation E140G on CaM structure and function. We present novel data showing that mutant-regulated CaMKIIδ kinase activity is impaired with a significant reduction in enzyme autophosphorylation rate. We report the first high-resolution crystal structure of a LQTS-associated CaM variant in complex with the CaMKIIδ peptide, which shows significant structural differences, compared to the WT complex. Furthermore, we demonstrate that the E140G mutation significantly disrupted Ca_v1.2 Ca²⁺/CaM-dependent inactivation, while cardiac ryanodine receptor (RyR2) activity remained unaffected. In addition, we show that the LQTS-associated mutation alters CaM's Ca²⁺-binding characteristics, secondary structure content, and interaction with key partners involved in excitation-contraction coupling (CaMKIIδ, Ca_v1.2, RyR2). In conclusion, LQTS-associated CaM mutation E140G severely impacts the structure-function relationship of CaM and its regulation of CaMKIIδ and Ca_v1.2. This provides a crucial insight into the molecular factors contributing to CaM-mediated arrhythmias with a central role for CaMKIIδ.

HIV-Tat Exacerbates the Actions of Atazanavir, Efavirenz, and Ritonavir on Cardiac Ryanodine Receptor (RyR2)

The incidence of sudden cardiac death (SCD) in people living with HIV infection (PLWH), especially those with inadequate viral suppression, is high and the reasons for this remain incompletely characterized. The timely opening and closing of type 2 ryanodine receptor (RyR2) is critical for ensuring rhythmic cardiac contraction-relaxation cycles, and the disruption of these processes can elicit Ca²⁺ waves, ventricular arrhythmias, and SCD. Herein, we show that the HIV protein Tat (HIV-Tat: 0-52 ng/mL) and therapeutic levels of the antiretroviral drugs atazanavir (ATV: 0-25,344 ng/mL), efavirenz (EFV: 0-11,376 ng/mL), and ritonavir (RTV: 0-25,956 ng/mL) bind to and modulate the opening and closing of RyR2. Abacavir (0-14,315 ng/mL), bictegravir (0-22,469 ng/mL), Rilpivirine (0-14,360 ng/mL), and tenofovir disoproxil fumarate (0-18,321 ng/mL) did not alter [³H]ryanodine binding to RyR2. Pretreating RyR2 with low HIV-Tat (14 ng/mL) potentiated the abilities of ATV and RTV to bind to open RyR2 and enhanced their ability to bind to EFV to close RyR2. In silico molecular docking using a Schrodinger Prime protein-protein docking algorithm identified three thermodynamically favored interacting sites for HIV-Tat on RyR2. The most favored site resides between amino acids (AA) 1702-1963; the second favored site resides between AA 467-1465, and the third site resides between AA 201-1816. Collectively, these new data show that HIV-Tat, ATV, EFV, and RTV can bind to and modulate the activity of RyR2 and that HIV-Tat can exacerbate the actions of ATV, EFV, and RTV on RyR2. Whether the modulation of RyR2 by these agents increases the risk of arrhythmias and SCD remains to be explored.

L-type Ca2+ channel recovery from inactivation in rabbit atrial myocytes

Adaptation of the myocardium to varying workloads critically depends on the recovery from inactivation (RFI) of L-type Ca²⁺ channels (LCCs) which provide the trigger for cardiac contraction. The goal of the present study was a comprehensive investigation of LCC RFI in atrial myocytes. The study was performed on voltage-clamped rabbit atrial myocytes using a double pulse protocol with variable diastolic intervals in cells held at physiological holding potentials, with intact intracellular Ca²⁺ release, and preserved Na⁺ current and Na⁺ /Ca²⁺ exchanger (NCX) activity. We demonstrate that the kinetics of RFI of LCCs are co-regulated by several factors including resting membrane potential, [Ca²⁺ ]_i , Na⁺ influx, and activity of CaMKII. In addition, activation of CaMKII resulted in increased I_Ca amplitude at higher pacing rates. Pharmacological inhibition of NCX failed to have any significant effect on RFI, indicating that impaired removal of Ca²⁺ by NCX has little effect on LCC recovery. Finally, RFI of intracellular Ca²⁺ release was substantially slower than LCC RFI, suggesting that inactivation kinetics of LCC do not significantly contribute to the beat-to-beat refractoriness of SR Ca²⁺ release. The study demonstrates that CaMKII and intracellular Ca²⁺ dynamics play a central role in modulation of LCC activity in atrial myocytes during increased workloads that could have important consequences under pathological conditions such as atrial fibrillations, where Ca²⁺ cycling and CaMKII activity are altered.

Multi-Scale Computational Modeling of Spatial Calcium Handling From Nanodomain to Whole-Heart: Overview and Perspectives

Regulation of intracellular calcium is a critical component of cardiac electrophysiology and excitation-contraction coupling. The calcium spark, the fundamental element of the intracellular calcium transient, is initiated in specialized nanodomains which co-locate the ryanodine receptors and L-type calcium channels. However, calcium homeostasis is ultimately regulated at the cellular scale, by the interaction of spatially separated but diffusively coupled nanodomains with other sub-cellular and surface-membrane calcium transport channels with strong non-linear interactions; and cardiac electrophysiology and arrhythmia mechanisms are ultimately tissue-scale phenomena, regulated by the interaction of a heterogeneous population of coupled myocytes. Recent advances in imaging modalities and image-analysis are enabling the super-resolution reconstruction of the structures responsible for regulating calcium homeostasis, including the internal structure of nanodomains themselves. Extrapolating functional and imaging data from the nanodomain to the whole-heart is non-trivial, yet essential for translational insight into disease mechanisms. Computational modeling has important roles to play in relating structural and functional data at the sub-cellular scale and translating data across the scales. This review covers recent methodological advances that enable image-based modeling of the single nanodomain and whole cardiomyocyte, as well as the development of multi-scale simulation approaches to integrate data from nanometer to whole-heart. Firstly, methods to overcome the computational challenges of simulating spatial calcium dynamics in the nanodomain are discussed, including image-based modeling at this scale. Then, recent whole-cell models, capable of capturing a range of different structures (such as the T-system and mitochondria) and cellular heterogeneity/variability are discussed at two different levels of discretization. Novel methods to integrate the models and data across the scales and simulate stochastic dynamics in tissue-scale models are then discussed, enabling elucidation of the mechanisms by which nanodomain remodeling underlies arrhythmia and contractile dysfunction. Perspectives on model differences and future directions are provided throughout.

3D bioprinting of complex tissues in vitro: state-of-the-art and future perspectives

The pharmacology and toxicology of a broad variety of therapies and chemicals have significantly improved with the aid of the increasing in vitro models of complex human tissues. Offering versatile and precise control over the cell population, extracellular matrix (ECM) deposition, dynamic microenvironment, and sophisticated microarchitecture, which is desired for the in vitro modeling of complex tissues, 3D bio-printing is a rapidly growing technology to be employed in the field. In this review, we will discuss the recent advancement of printing techniques and bio-ink sources, which have been spurred on by the increasing demand for modeling tactics and have facilitated the development of the refined tissue models as well as the modeling strategies, followed by a state-of-the-art update on the specialized work on cancer, heart, muscle and liver. In the end, the toxicological modeling strategies, substantial challenges, and future perspectives for 3D printed tissue models were explored.

Ventricular voltage-gated ion channels: Detection, characteristics, mechanisms, and drug safety evaluation

Cardiac voltage-gated ion channels (VGICs) play critical roles in mediating cardiac electrophysiological signals, such as action potentials, to maintain normal heart excitability and contraction. Inherited or acquired alterations in the structure, expression, or function of VGICs, as well as VGIC-related side effects of pharmaceutical drug delivery can result in abnormal cellular electrophysiological processes that induce life-threatening cardiac arrhythmias or even sudden cardiac death. Hence, to reduce possible heart-related risks, VGICs must be acknowledged as important targets in drug discovery and safety studies related to cardiac disease. In this review, we first summarize the development and application of electrophysiological techniques that are employed in cardiac VGIC studies alone or in combination with other techniques such as cryoelectron microscopy, optical imaging and optogenetics. Subsequently, we describe the characteristics, structure, mechanisms, and functions of various well-studied VGICs in ventricular myocytes and analyze their roles in and contributions to both physiological cardiac excitability and inherited cardiac diseases. Finally, we address the implications of the structure and function of ventricular VGICs for drug safety evaluation. In summary, multidisciplinary studies on VGICs help researchers discover potential targets of VGICs and novel VGICs in heart, enrich their knowledge of the properties and functions, determine the operation mechanisms of pathological VGICs, and introduce groundbreaking trends in drug therapy strategies, and drug safety evaluation.

CaMKII inhibition has dual effects on spontaneous Ca2+ release and Ca2+ alternans in ventricular cardiomyocytes from mice with a gain-of-function RyR2 mutation

In conditions with abnormally increased activity of the cardiac ryanodine receptor (RyR2), Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) can contribute to a further destabilization of RyR2 that results in triggered arrhythmias. Therefore, inhibition of CaMKII in such conditions has been suggested as a strategy to suppress RyR2 activity and arrhythmias. However, suppression of RyR2 activity can lead to the development of arrhythmogenic Ca²⁺ alternans. The aim of this study was to test whether the suppression of RyR2 activity caused by inhibition of CaMKII increases propensity for Ca²⁺ alternans. We studied spontaneous Ca²⁺ release events and Ca²⁺ alternans in isolated left ventricular cardiomyocytes from mice carrying the gain-of-function RyR2 mutation RyR2-R2474S and from wild-type mice. CaMKII inhibition by KN-93 effectively decreased the frequency of spontaneous Ca²⁺ release events in RyR2-R2474S cardiomyocytes exposed to the β-adrenoceptor agonist isoprenaline. However, KN-93-treated RyR2-R2474S cardiomyocytes also showed increased propensity for Ca²⁺ alternans and increased Ca²⁺ alternans ratio compared with both an inactive analog of KN-93 and with vehicle-treated controls. This increased propensity for Ca²⁺ alternans was explained by prolongation of Ca²⁺ release refractoriness. Importantly, the increased propensity for Ca²⁺ alternans in KN-93-treated RyR2-R2474S cardiomyocytes did not surpass that of wild type. In conclusion, inhibition of CaMKII efficiently reduces spontaneous Ca²⁺ release but promotes Ca²⁺ alternans in RyR2-R2474S cardiomyocytes with a gain-of-function RyR2 mutation. The dominant effect in RyR2-R2474S is to reduce spontaneous Ca²⁺ release, which supports this intervention as a therapeutic strategy in this specific condition. However, future studies on CaMKII inhibition in conditions with increased propensity for Ca²⁺ alternans should include investigation of both phenomena.NEW & NOTEWORTHY Genetically increased RyR2 activity promotes arrhythmogenic Ca²⁺ release. Inhibition of CaMKII suppresses RyR2 activity and arrhythmogenic Ca²⁺ release. Suppression of RyR2 activity prolongs refractoriness of Ca²⁺ release. Prolonged refractoriness of Ca²⁺ release leads to arrhythmogenic Ca²⁺ alternans. CaMKII inhibition promotes Ca²⁺ alternans by prolonging Ca²⁺ release refractoriness.

Role of Reduced Sarco-Endoplasmic Reticulum Ca2+-ATPase Function on Sarcoplasmic Reticulum Ca2+ Alternans in the Intact Rabbit Heart

Sarcoplasmic reticulum (SR) Ca²⁺ cycling is tightly regulated by ryanodine receptor (RyR) Ca²⁺ release and sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA) Ca²⁺ uptake during each excitation–contraction coupling cycle. We previously showed that RyR refractoriness plays a key role in the onset of SR Ca²⁺ alternans in the intact rabbit heart, which contributes to arrhythmogenic action potential duration (APD) alternans. Recent studies have also implicated impaired SERCA function, a key feature of heart failure, in cardiac alternans and arrhythmias. However, the relationship between reduced SERCA function and SR Ca²⁺ alternans is not well understood. Simultaneous optical mapping of transmembrane potential (V_m) and SR Ca²⁺ was performed in isolated rabbit hearts (n = 10) using the voltage-sensitive dye RH237 and the low-affinity Ca²⁺ indicator Fluo-5N-AM. Alternans was induced by rapid ventricular pacing. SERCA was inhibited with cyclopiazonic acid (CPA; 1–10 μM). SERCA inhibition (1, 5, and 10 μM of CPA) resulted in dose-dependent slowing of SR Ca²⁺ reuptake, with the time constant (tau) increasing from 70.8 ± 3.5 ms at baseline to 85.5 ± 6.6, 129.9 ± 20.7, and 271.3 ± 37.6 ms, respectively (p < 0.05 vs. baseline for all doses). At fast pacing frequencies, CPA significantly increased the magnitude of SR Ca²⁺ and APD alternans, most strongly at 10 μM (pacing cycle length = 220 ms: SR Ca²⁺ alternans magnitude: 57.1 ± 4.7 vs. 13.4 ± 8.9 AU; APD alternans magnitude 3.8 ± 1.9 vs. 0.2 ± 0.19 AU; p < 0.05 10 μM of CPA vs. baseline for both). SERCA inhibition also promoted the emergence of spatially discordant alternans. Notably, at all CPA doses, alternation of SR Ca²⁺ release occurred prior to alternation of diastolic SR Ca²⁺ load as pacing frequency increased. Simultaneous optical mapping of SR Ca²⁺ and V_m in the intact rabbit heart revealed that SERCA inhibition exacerbates pacing-induced SR Ca²⁺ and APD alternans magnitude, particularly at fast pacing frequencies. Importantly, SR Ca²⁺ release alternans always occurred before the onset of SR Ca²⁺ load alternans. These findings suggest that even in settings of diminished SERCA function, relative refractoriness of RyR Ca²⁺ release governs the onset of intracellular Ca²⁺ alternans.

Novel PITX2 Homeodomain-Contained Mutations from ATRIAL Fibrillation Patients Deteriorate Calcium Homeostasis

Atrial fibrillation (AF) is the most common cardiac arrhythmia in the human population, with an estimated incidence of 1–2% in young adults but increasing to more than 10% in 80+ years patients. Pituitary Homeobox 2, Paired Like Homeodomain 2 (PITX2c) loss-of-function in mice revealed that this homeodomain (HD)-containing transcription factor plays a pivotal role in atrial electrophysiology and calcium homeostasis and point to PITX2 as a candidate gene for AF. To address this issue, we recruited 31 AF patients for genetic analyses of both the known risk alleles and PITX2c open reading frame (ORF) re-sequencing. We found two-point mutations in the homedomain of PITX2 and three other variants in the 5’untranslated region. A 65 years old male patient without 4q25 risk variants but with recurrent AF displayed two distinct HD-mutations, NM_000325.5:c.309G>C (Gln103His) and NM_000325.5:c.370G>A (Glu124Lys), which both resulted in a change within a highly conserved amino acid position. To address the functional impact of the PITX2 HD mutations, we generated plasmid constructs with mutated version of each nucleotide variant (MD4 and MD5, respectively) as well as a dominant negative control construct in which the PITX2 HD was lacking (DN). Functional analyses demonstrated PITX2c MD4 and PITX2c MD5 decreased Nppa-luciferase transactivation by 50% and 40%, respectively, similar to the PITX2c DN (50%), while Shox2 promoter repression was also impaired. Co-transactivation with other cardiac-enriched co-factors, such as Gata4 and Nkx2.5, was similarly impaired, further supporting the pivotal role of these mutations for correct PITX2c function. Furthermore, when expressed in HL1 cardiomyocyte cultures, the PITX2 mutants impaired endogenous expression of calcium regulatory proteins and induced alterations in sarcoplasmic reticulum (SR) calcium accumulation. This favored alternating and irregular calcium transient amplitudes, causing deterioration of the beat-to-beat stability upon elevation of the stimulation frequency. Overall this data demonstrate that these novel PITX2c HD-mutations might be causative of atrial fibrillation in the carrier.